content charge on internet account

I had internet contract with mtn since 2012 unlimited where i pay R314 monthly only for internet and i only use it for internet. Some months back mtn charged me extra claiming i have sent out an sms i tried to query it but i let it go. Now they charged me almost double the agreed amount of R314 claiming that there are some content charge where some WASPA company they charged me. My query is that the agreement with mtn is only for internet nothing more, if some company claim airtime from me, i believe mtn should have consulted me to check why i choose option for airtime to be debited on my account whereas the agreement is only for unlimited internet or declined that request from that company. I believe mtn in some how they do get profit from this company and that's why they let other companies do as they like on our account. Now my option is to reverse this money debited from me and pay for what i agreed with mtn.